Any advice anyone has would be much appreciated. I am at the end of my rope right now, and I just don't know enough to proceed. I am trying to educate myself to make a better decision, and if anyone knows anything, I would really appreciate hearing. She is a 2005 Toyota Prius with 103k miles, and was previously in great working condition.

Background: I am a female in the South, and have not had the best luck with mechanics. I often feel as though my lack of knowledge is being taken advantage of.

-SPRING 2013: I got hit by a closing fence that caused damage to my rear bumper. The car ran fine, but the back bumper and the under panels would need to be replaced eventually. Since I am moving for a year to a new city, in which I did not need a car, we put off the repair, since we felt it would be expensive

FALL 2013- The car is stored and driven once every 3 weeks, by one of my siblings. No problems.

WINTER 2013- My parents take all cars, including mine to get oil changes.

-Spring 2014- My sister's car is totaled, she borrows my car. At some point, she said she felt like the engine was chugging a bit, and the check engine light came on. She took it to a mechanic, who said that there was NO OIL in the car. Confirmed that the engine was fine, refilled the oil. No other problems. Switched to synthetic oil. Puts about 14k miles on the car.

May 2014- Car is taken to get the bumper repaired. Repair takes 3 weeks (2 weeks longer than quoted). We get the car back, I drive it 15 miles, and the Master Warning Light comes on. I call the repair shop, and they tell me that they had forgotten to tell me that the battery had died while in the shop (no explanation offered) and that if I drive it around, the light should reset. They assure me they had only done cosmetic work (Right back passenger side bumper), and were not in the engine. The light does not reset, and I call back. They say to bring it back in. I bring it back, and the shop manager will only keep saying that it's not their fault. I am adamant, and insist they take the car back and solve the problem. They take the care back, come back out 15 minutes later, and the light is off. I ask if it is fixed, and they confirm the light is off. I ask again, what is wrong, they say there is no error code associated with the light, that they reset the light. 10 miles later the light is back on.

At this point I take the car to Toyota. They say the error code associated with the light is for the battery cooling system, and they test the system and the system is fine- it is a false error code. They say that the bumper replacement behind the right passenger side has a sensor that was most likely bumped or moved. They reset and send me on my way. At this point the brake system warning light is also going off and on, while the Master Warning light remains constant. 10 miles later, the light is back on. Back to Toyota. This time, they take the cooling system apart, find a rusted wire, replace, check the cooling system, alls well- false error code, send me on my way. 2 days go by. Light comes back on. At that point, I've been told by Toyota its a false code, this has eaten up 2 weeks of in and out, I accept that the code is false, maybe the sensor is off. 1 more week with the light on, the light goes off, and stays off for 5 weeks, including a 500 mile trip through the south in 97 degrees.

July 2014- Light comes off and on at random. No problems with the car, no worries, but I do notice the light only comes on during the days when it is HOT, and goes off when the temperature drops below 90 and then sometimes just goes off randomly- even in the high heat. 2nd to last week in July one day I went to the grocery, about a mile away, when I came out, I started the car and the engine, chugged a but, making the car shake. Stopped after I got out of the parking lot. Strange, but lasted 15 seconds, and never happened again, until....

Yesterday- Driving back to my parents from my SO's house (500 mile trip) No lights, no indicators, all is well. 310 miles in, stop to top off, let our dog walk. The engine starts chugging again. Does not stop, check engine light comes on. Only happens when going slow. Above 50 mph, no problem. Finish the drive, straight to Toyota. I'm at 103k miles, still have 5k until my next oil change. Informed him about the former oil problem, worried I have a leak in my oil line, or my fuel filter is clogged. Toyota calls me today- my battery system is failing. He said that all of the codes are associated with the battery. It makes me wonder if they have tested the battery at all or is there another false code with the battery? CAN THE BATTERY CAUSE THAT CHUGGING? In all of my research, it says spark plug or fuel filter, NOT BATTERY as a cause.

At this point, I have no idea what to believe. The first time, they tested my battery and ASSURED me it was not my battery in June. It was something we were really worried about being the case because the battery is a 4k repair, so he tested the hell out of the battery. Can the battery have gotten that bad, that quickly?

Is it possible that its not the battery at all, but just the bad sensor, and just that the fuel filter is clogged? Would the battery cause that chugging? There doesn't appear to be any problems starting the car at all.

Is it possible that the battery cooling system was bad all along and that caused the battery to fail? If so, then I would need to fix the cause and the battery failure is just a symptom.

This is a $4200 repair, and I LOVE my car, but being honest, I don't think the car is worth that. That being said, I need a car, and I'm not sure I can take on new payments right now. The car I could afford in the same as repair range runs a risk of failure soon as well, and I can't keep replacing cars. Does anyone have any experience with this? I just need to figure out which path to commit to. I just can't believe all of this is independently happening at once, but maybe I am paranoid. I don't trust any of these mechanics at this point, and I want to make the best decision.

UPDATE: Thank you everyone for all of your advice. I went in and had the 12v battery checked, as well as the computer. There was some clarification as well- it was both the main battery and the spark plugs that have gone out. Ultimately, 2 weeks later, we decided to put the repair payment towards a new car instead. The city I am in being a driving city, and being a small (5'0, 110lbs female) both my fiancee and my parents put pressure for a new-damn-sure-it-won't-break-down car. Thank you so much for your advice. I think the mechanic was surprised when I went back with a plan of attack and some difficult questions. I was pretty frustrated and overwhelmed with the whole experience, and you guys helped in a big way.

Did anyone check for a low or bad 12v battery? The traction battery is the big expensive one. But the 12v still runs the computer and the car goes really wonky when it starts to go out. Toyota should have checked that.

The traction battery cooling system is just a fan behind the little grill on your rear passenger seat. It should be easy for them to tell if it's working or not. Did any codes ever point to the battery, because faulty cells will throw a code. I'd imagine there's a thermal code as well for overheating specifically.

The ICE runs like any other 4cyl engine; so if it's rough I wouldn't look at the hybrid system except maybe in the trans-axle that combines the power. New plugs would be cheap and I replaced mine after 100k miles on my 2006. You'd want that anyway.

I wouldn't jump to replacing to the battery pack unless the car says it needs replaced. If the cooling fan is running then you might just have a bad sensor somewhere in the system. Might have more issues hidden like the rusted wires they replaced.

I am sympathetic about this. I HATE electronic problems nobody can seem to fix, but expect us to throw cash at each attempt. I had a Grand Caravan I liked but the entire electronic system, including headlights, would just die for no reason. Nobody could fix it after I paid a lot out of pocket in attempts. I junked it for a few hundred to get it out of my life.

This is not true. For the Gen2 and Gen3 Prius, the compressor is electric and runs off the high voltage (AKA traction) battery. You won't get any A/C without the car turned on.

Pretty much the only thing the 12 volt battery does is power the computers and the interior lights. When you "start" the car, the computers use the high voltage battery to make a strong 12 volt power source that charges up the 12 volt battery. Thats why the interior lights get brighter when you start a Prius that has a low 12 volt battery.

Fun fact: if you know what you are doing, it is possible to "jump start" a Prius from a 12 volt drill battery.

Yep, if your 12 volt battery (the little one in the hatch in the back on the passenger side) is low voltage, it will cause all kinds of strangeness. If it is more than 5 years old, replace it straight away.

The 12 volt battery in the prius is pretty finicky; if it gets run down a few times it will stop holding a proper charge. It is very easy to kill the 12 volt battery by leaving the door open (and thus the interior lights on) overnight a few times. This is likely what the guys at the repair shop did.

They can be expensive if you buy from the dealer and have the dealer do the install. If you can install it yourself, you can save around $100.

Second going to priuschat.com Let them know where you are (more specifically). They may have some ideas about what else may be going on, and where to take the vehicle.

If it really is the traction battery (and it may well be: this is a 9 year old vehicle with over 100k miles in an early model) you have a decision to make. I don't know if I would drop $4200 on a new battery, but that is a fraction of a new car, the vehicle is paid for, and appears to be otherwise running well. It's a decision that you will have to make.

There may be some cheaper options: use of remanufactured batteries will decrease the total cost.

My car would beep on shut off when the battery would be DOA the next morning. The first morning it did it, when I came out to a dead car after school I thought I might have left the car running.... (I am NOT a morning person!). The second time, I knew what to expect the next morning and had AAA waiting to jump it - which became a tow and battery replacement.

My husband's '09 wouldn't start at all in cold temperatures when his battery was going out, and would trip the master warning light.

I would add that the 12V battery is very small, smaller than the one in most economy cars, and is located in the rear passenger side fender area in other words, very close to where your body shop was working. As noted above, this little battery powers the computer that runs everything.

First check your records; if the 12V battery has never been replaced, just replace it. Don't get it tested; it's too old, just replace it. If intermittent problems continue, I'd look for an auto-electric shop and ask them to check the wiring between the 12V battery in the back, and the fuse block in the front under the hood. I can well imagine the body shop guys pinching a cable between two body parts or something, and creating an intermittent problem.

I would guess the break error code was due to the capacitor pack being unplugged during the bumper repair and then never plugged back in. It's a black box back next to the 12 volt battery that provides "emergency" power to the brake system in the case of a total electrical failure.

In general, codes don't lie. Either the problem is present or the thing that senses the problem has gone bad.

It would be helpful to have the complete codes from the dealership. Sometimes there are extra parts to the codes (maybe called snapshot data?).

The codes will tell you (or the fine people over at www.priuschat.com) what the car says is wrong.

The high voltage (AKA traction) battery doesn't like high heat (which is why there is a cooling fan system back there). If the batteries are running hot (like driving in 90+ tems with the windows down and the AC off) it will shorten the life of the high voltage battery.

Don't feel like the only option for HV battery replacement is the dealership. There are 3rd parties that sell replacement batteries for about half the cost of a dealership. Some people DIY and buy a HV battery back from a wrecked prius in a junk yard and install that. This is an inexpensive way to go, but it takes some effort and skill.

There are options. www.priuschat.com is your friend(s). Lots of info there and they are always willing to answer new questions.

It really depends on the outdoor conditions. Since the A/C is cooling down the cabin, the intake air that cools the batteries gets cooled as well. If it's super hot outside, it's probably worth the little bit of extra gas to cool down the cabin.

except technically its using the battery to cool down the cabin. Due to inefficiencies, i don't think its possible to cool down the battery with AC while using the battery to run the AC. Its not so much about saving gas as it does it make sense?

A good theoretical example of this might be to imagine running on a treadmill that powers an AC unit, Now blow the AC unit air directly on your body to cool you as you run, will the human powered AC unit cool you down More than running itself will heat you up? (all power provided to this system is human power must be greater than heat generated) Intuition says no to me.

I could probably do some calculation to figure it out, but basically the Heat generated by the Battery to use the AC must be less than the Heat/time removed from the cabin. (there are other inefficiencies as well as your cabin actually accumulates a lot of heat from solar through the windows, extra body heat.. making battery cooling via C much less realistic).

The only thing that clearly makes sense is the battery cooling fan which uses very little power but removes alot of heat from the battery

You're not accounting for the AC radiator outside the cabin that transfers the heat extracted from the air to the outside world. The waste heat from the A/C doesn't go back into the cabin.

It's a pretty simple system. Energy goes in (in the form of gasolene). Cold comes out into the cabin and heat goes out to the outside air.

Also, the A/C would often be directly powered from the motor/generators when you are in a regen braking situation.

Really, it's a matter of scale. In my experience, the A/C pulls somewhere between 1 and 5 amps of power. Normal operations during driving regularly see 75 amps (in or out). Really, the small extra load of the A/C compressor doesn't make a dent in what the HV battery system normally experiences. The battery cooling system is built to deal with big amps.

Very little of the energy coming out of the battery is waste heat, so yes the increased draw on the battery of the ac will be not overpower the cooling effect. There is inefficiency and fuel consumption will suffer, but it will still cool the battery.